Methods and apparatus of the aforesaid type are known in the art in various forms, for example from DE-A-38 17 357 and from the company periodical "POLYSIUS TEILT MIT" No. 171, 1992, pages 8 and 9 (illustrations 15 and 16).
A significant idea in these known embodiments concerns the mode of operation and the construction of the calcination zone in such a way that--also in connection with fuels which are slow to react or difficult to burn--apart from the most thorough calcination possible of the preheated material a marked reduction in the nitrogen oxides (NO.sub.X) contained in the kiln exhaust gases can be brought about. For this purpose a quantity of tertiary air is delivered to the reaction chamber within the calcination zone such that a reducing atmosphere is maintained therein. Furthermore the preheated material coming from the second-lowest cyclone stage of the cyclone preheater is divided into at least two part-quantities, of which one first part-quantity is fed into the lower region of this reaction chamber and a second part-quantity is fed into the upper region thereof in order to adjust the reaction temperature in the reaction chamber to a desired temperature which is generally somewhat higher than the usual calcination temperature. In particular the aforementioned division of the preheated material frequently leads to structural problems which have a particularly noticeable disadvantageous effect in the case of subsequent conversion of such heat treatment apparatus.
The object of the invention is to improve the method and apparatus in such a way that with good adjustability of the reaction temperature in the reaction chamber a very thorough reduction of the nitrogen oxides (NO.sub.X) can be ensured with a relatively simple and space-saving construction.